This invention relates to stringing racquets, specifically a means for measuring the actual tension of the strings in a strung racquet.
To illustrate the current situation of measuring the tension of the strings in a strung racquet it is appropriate to quote the 1992-1993 Stringer's Digest, a publication of the U.S. Racquet Stringers Association.
Dictionary of Terms: PA0 ACTUAL TENSION: The tension of the strings in the strung racquet. Actual tension will consistently be lower than REFERENCE TENSION (the tension commonly specified by racquet manufacturers and players) due to numerous variables--such as ELONGATION and friction. Actual tension can be less than the reference tension by as much as 5-25%. Actual tension will continue to decrease over a period of time due to a strings' stretch, loss of resilience and wear.
Shortly after stringing, the actual tension will depend on the stringer's technique, the characteristics of the stringing machine, and the properties of the string being used. If the actual tension can be as much as 25% less than the reference tension, a reference tension of 27 kg (60 lb) could result in an actual tension as low as 20 kg (45 lb). This is a big variation and would have a noticeable impact on even a beginner's game. Experienced players would like to control their racquet tension to within 1.4 to 2.3 kg (3 to 5 lb). It would be extremely valuable to be able to measure the actual tension after the racquet has been strung so that this variable could be eliminated for the players.
After some use, because of a natural loss of tension and elasticity, a racquet will eventually need restringing, even if a string has not broken. A frequently heard guide is "to restring as many times in a year as the racquet is played with in a week". This is obviously a very rough guide. Many factors will influence the need for new strings. For example, a hard hitter will cause faster deterioration in the strings than a soft hitter, and strings are made in a range of quality, gauge, strength, endurance, and elasticity. Clearly a measure of the actual tension would give a good indication of the time to restring.
Current string tension testing devices do not measure the actual tension. There are three basic ways in which existing devices get a measurement which is related to, but not the same as the actual tension:
(a) a measurement of the force required to deflect the string face a predetermined distance (or the distance deflected by a predetermined force)
(b) a measurement of the torque required to deflect a short length of string through a predetermined angle (or the angle deflected by a predetermined torque).
(c) a measurement of the string's natural frequency of vibration.
All of these methods are influenced by the string elasticity. One manifestation of method (a) has a means of (roughly) restricting the deflection to a standard length of string, but (b) and (c) will depend on the string length. Methods (a) and (c) will be influenced by the string pattern and the friction between strings. The best that can be said about these methods is that they will give a relative indication of the tension.
Accordingly there is a real need for a device which measures the actual tension.
Accordingly, several objects and advantages of my invention are:
(a) to provide a direct measurement of the actual tension.
(b) to provide a measure of the tension in any particular string
(c) to provide a measure of string tension which does not depend on the string properties such as its length, or its elasticity.
(d) to provide a measure of string tension which does not depend on the size of the racquet, or the density of stringing,
(e) to provide a measure of string tension which can be used directly to decide whether or not the racquet needs to be restrung.
(f) to provide a measurement device which can be made in various sizes, and adjustable, so that the tension on almost any string on a tennis, racquetball, squash, badminton, or any other racquet could be measured.
Further objects and advantages of my actual tension measuring device will become apparent from a consideration of the drawings and ensuring description.